1. Field of the Invention
The present invention relates to multimedia communication, and more particularly, to an apparatus and method for efficiently transmitting a multimedia data stream according to the state of a network and the type of a video frame.
2. Description of the Related Art
Streaming is a technology to process data to be transmitted so that data transmission can take place endlessly and continuously. Accordingly streaming has become increasingly important in line with the growth of the Internet, because most users do not have Internet connection lines fast enough to rapidly download voluminous multimedia files. When streaming is used, data can be presented by a client browser or plug-in even before full transmission of a file is completed.
For streaming over a wireless network, it is necessary to solve problems such as a variable bandwidth, a multi-path fading phenomenon, movement, handoff, traffic, partial data loss due to packetization of a video frame, and variety of client devices.
As an approach to solve the problems, a cross-layer optimization method has been suggested. The cross-layer optimization method is based on the exchange of layers' information between at least two protocol stacks.
A video stream consists of frames including picture information. Generally, the frames include various types of video information according to video compression methods. According to a moving picture expert group (MPEG) video compression method, an intra-coded (I) frame including information of the entire screen without reference to other frames, a bidirectional-coded (B) frame including information of a difference from a past frame and/or a future frame, and a predictive-coded (P) frame coded with reference to information of a past I or P frame are used. The I frame has a large size, and the B frame or P frame has a small size and is transmitted several times between transmission of the I frame. One I frame and a plurality of B frames and P frames following the I frame form a group of pictures (GOP).
The I frame can be positioned anywhere in a video stream, is used for a random data access, and is coded without reference to other frames. The P frame is coded or decoded using information of a past I frame and a past P frame. The B frame is coded or decoded using information of past and future I frames and past and future P frames. An I frame loss has an influence upon the other B frames and P frames in a GOP. A P frame loss has an influence only upon the B frames in a GOP. A B frame loss has no influence upon other types of frames.
Therefore, to maintain the quality of an audio/video (AV) service in a poor channel environment, the transmission success probability of an I frame should be improved when compared to a B frame or a P frame. However, the characteristic of an MPEG frame is not reflected in a conventional system where transmission control parameters are set statically. A conventional multimedia transmission system processes an I frame and other types of frames in the same manner when a transmission error occurs, resulting in degradation of a service quality.